1. Field of the Invention
The present invention generally relates to a method and apparatus for forming an image using a photosensitive medium in the form of a screen having therein a plurality of fine openings which is hereinafter referred to also merely as a screen.
More particularly, the present invention is directed to a novel means for producing a non-image area on the modulated member at the time of modulation in the image forming process using the above-mentioned screen.
2. Description of the Prior Art
The screen used in the invention is, for example, a multilayer structure composed of photo-conductive material and electrically conductive material and, optionally, insulating material having a plurality of fine openings formed therein. Various processes of forming an image by using such a screen have been known in the art. An example of the image forming process is of the type in which a primary electrostatic latent image is formed on the screen by initially charging the screen with electric charge using a corona discharger or the like and then projecting a light image of an original on the screen, and a secondary electrostatic latent image is formed on an electrifiable member by an ion stream controlled under the action of an electric field formed on the screen openings in accordance with the primary electrostatic latent image on the screen. This type of image forming technique is disclosed in U.S. Pat. No. 3,645,614 and generally called ion stream control type of electrophotographic method. The secondary electrostatic latent image formed on the electrifiable member in the above described manner can be developed and fixed using a conventional developing device and fixing device.
One of the important problems involved in the image forming apparatus using such screen photosensitive medium is the scattering of toner. Scattered toner often blocks the openings of the screen which is usually in the order of several hundreds mesh. If blocking occurs, the modulation ability of the screen is substantially reduced or the screen is damaged by the phenomenon of leakage occurring when a bias voltage is applied. The trouble caused by toner scattered within the main body of the apparatus may be prevented to some extent by providing means for preventing scattering of toner at some toner treating stations such as the developing station and cleaning station. However, even when such prevention means is employed, it is impossible to effectively prevent toner from being scattered on a copy sheet which has not been developed yet. This problem will be more concretely described hereinafter with reference to a copying machine of the type in which a latent image is formed on a dielectric drum by imagewise modulating an ion stream with the aid of the above described screen, wherein the latent image is developed with toner, the toner image is then transferred onto a sheet member such as transfer paper and the transferred toner image is fixed on the sheet by heat roller fixing means.
As is well known to those skilled in the art, in the above mentioned copying machine there are some steps at which a wind rises. At the transfer station, a wind toward the front side of the sheet rises when the fore end of the sheet is electrostatically drawn to the surface of the drum. Also, after transferring a wind rises at the front part of the sheet when the latter is stripped from the drum surface. Such wind disperses the toner on the fore end of the sheet and makes the fore edge of the sheet dirty. Furthermore, at the time of the dirty sheet being conveyed through the apparatus, the sheet is subjected to vibration which causes a scattering of the toner from the fore edge portion of the sheet. It is a matter of course that the quality of the copy is substantially reduced by such dirt toner on the fore end part of the sheet. In the worst case, at the fixing station using heat roller fixing means, the sheet may be caught into the fixing roller due to the toner adhered onto the fore edge of the sheet.
To separate a sheet from the dielectric drum, belt or pawl is widely used. But, such sheet separation means also constitutes a source of generation of scattered toner. When such sheet separation means comes into contact with the toner on the drum surface, the toner is sometimes scattered.
The most effective solution to the above mentioned problem of toner scattering is that no image be formed at the problematical portions such as the fore end part of the image area on the dielectric drum and the side edge portions thereof at which sheet separation means works. In other words, the solution is to intentionally form a non-image portion within the area where an image can otherwise be formed. Some methods to realize the solution have been proposed and already used.
A representative of the known method comprises irradiating light on a limited area of the screen which corresponds to the area at which it is desired to form a non-image area. Conventionally, this method is carried out by projecting a lamp light directly on the subject area of the screen. However, it is impossible to form satisfactorily sharp boundaries employing this method because of diffusion of the irradiated light. It is true that this method is somewhat effective in forming a sharp boundary at the side part of the image area. But, it is practically impossible to exclusively irradiate light at only the fore end and/or the rear end of the image area. Therefore, a sufficiently sharp boundary can scarcely be formed at the fore- and rear end parts by this method. Another disadvantage of this known lamp method is found in the increase of electric power consumption caused by the additional lamp required for forming a non-image area. Taking into consideration the fact that the photosensitive medium used therein is in the form of a screen having a plurality of fine openings and that light is diffused on the screen, such irradiation of light to the screen is undesirable, apart from other problems involved in the known method.
Another method proposed to form a non-image area without irradiation of light on the screen is that in the developing device there is provided a limiting member for limiting the area to be developed by the developing device. However, this method is not versatile. It is difficult to form a non-image area exclusively at the fore edge portion of the image area or to exchange the non-image area to be formed from one to another at the user's will.